Ethylene (on left in grey) is usually contaminated with acetylene (blue), which can ruin the process that converts ethylene into polyethylene. SIFSIX MOFs (center) can capture the acetylene efficiently, leaving pure ethylene (right). Image: Zhou/NIST.Plastic manufacturing is an energy-intensive process, but new research performed in part at the US National Institute of Standards and Technology (NIST) has revealed a way to reduce this energy demand by taking advantage of a class of materials that can efficiently filter impurities.
The findings, published in a recent paper in Science, show that porous materials known as metal-organic frameworks (MOFs) can effectively remove the contaminant acetylene from ethylene, the basic building block of the widely-used plastic polyethylene. The research suggests that filtering out acetylene using MOFs would produce ethylene at the high purity that industry demands while sidestepping the current need to convert acetylene to ethylene via a costly catalytic process.
Polyethylene is the most widely-used plastic in the world, which makes ethylene the most widely produced organic compound in the world, with well over 100 million tons of it manufactured each year, largely by refining crude oil.
Newly-made ethylene is not pure enough to be converted directly into polyethylene, because the refinement process also creates a substantial amount of acetylene, which can ruin the catalysts that string ethylene molecules together to produce polyethylene. The conventional industrial solution is to convert this undesirable acetylene into ethylene as well, but this step requires the use of an expensive palladium catalyst and consumes a significant amount of energy.
The research team, which included scientists from the NIST Center for Neutron Research (NCNR) and five universities from around the world, found that a family of MOF materials called SIFSIX, discovered in the 1990s, might offer a better way to remove the acetylene. MOFs are porous crystals that under a microscope look a bit like a building under construction – lots of girders with space in between. The SIFSIX group gets its name from some of its girders, which are formed from silicon (Si) and six atoms of fluorine (F6).
The team found that when they passed ethylene through the MOFs, the fluorine attracted and captured most of the acetylene contaminant, but let the now-purified ethylene pass unhindered. Varying the size of the pores by changing the length of the girders allowed the MOFs to remove acetylene from ethylene at concentrations ranging from between 1% to 50%, which are typical in industry.
The SIFSIX MOFs set records among adsorbent materials for both selectivity (capturing the acetylene rather than the ethylene) and adsorption capacity. According to the research team, the results show that the SIFSIX group offers a viable alternative to standard industrial practice.
"They reduced the amount of acetylene in ethylene down to less than 2 parts per million (ppm), which is lower than the 5ppm that polyethylene manufacturing requires," said NIST materials scientist Wei Zhou. "SIFSIX MOFs are easy to produce, safe to use, and can be reused over and over again. They also have the advantage of being stable, which is not true of all MOFs."
This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.